1. Field of the Invention
This invention relates to a solid-state image pickup device, and more particularly to a CMOS image sensor used for a mobile-phone with an image sensor, a digital camera, or a video camera.
2. Description of the Related Art
A wide variety of arrangements of color filters used in image sensors, ranging from complementary color filters to primary-color Bayer arrangements, have been proposed together with a method of processing the signals. With the further microfabrication of pixels in recent years, image sensors with pixels of the order of 2 μm have been put to practical use and the development of 1.75-μm pixels and 1.4-μm pixels is now in progress. In microscopic pixels of the order of 2 μm or less, since the quantity of incident light decreases significantly, deterioration by noise is liable to take place. In this connection, as a method of improving the sensitivity of microscopic pixels, an image sensor using a white (W) color filter has been proposed (e.g., refer to Jpn. Pat. Appln. KOKAI Publication No. 8-23542, Jpn. Pat. Appln. KOKAI Publication No. 2003-318375, Jpn. Pat. Appln. KOKAI Publication No. 2004-304706, or Jpn. Pat. Appln. KOKAI Publication No. 2005-295381).
However, since white (W) pixels are highly sensitive, the white (W) signals obtained from the W pixels get saturated easily. In addition to this problem, since Y signal (luminance signal)=W signal, there is a problem with color reproducibility. Normally, the color reproducibility of the RGB signals produced from the YUV signals becomes worse unless the Y signal is generated at this ratio: Y=0.59G+0.3R+0.11B. Furthermore, in the above patent documents, an effective signal process using W pixels has not been carried out.